1. Field of the Invention
This invention relates to apparatus for plasma processing of substrates, and more particularly to subtractive (etching) and additive (deposition) processing of electronic circuit chips and packaging materials.
2. Technical Problem
The problem with capacitive coupling of R.F. energy to a plasma employed for etching or depositing films is that to increase power to the level required to generate the plasma required, the voltage will be so high that the charged particles in the plasma will be accelerated to an excessive level of kinetic energy and will tend to sputter the work piece and to etch or sputter away any masks. The effect will be to chamfer the mask opening, i.e. increase the size of openings in masks by etching the edges of the masks. The effect also leads to ion damage and loss of selectivity. This is unacceptable as the requirements in the art are to decrease the size of openings as dimensions are decreasing in microelectronics. Instead one would like the flexibility of varying the ion energy according to the desired process.
2. Related Art
U.S. Pat. No. 3,705,091 of Jacob for "Gas Discharge Apparatus" shows a cylindrical glass reaction chamber coaxially wound with a helical R.F. coil energized by high frequency (13.5 MHz) R.F. to generate a plasma in a vacuum for etching of a tray of semiconductor slices The system operates in the 1 Torr pressure range and produces mainly reactive radicals. The Jacob system does not operate in the desired reactive ion etching, RIE mode of this invention. In the pressure range desired for the present invention of 1 to 50 mTorr, the Jacob system would produce very non-uniform and very slow etching No means for confining the plasma is shown
M. C. Vella, K. W. Ehlers, D. Kippenhan, P. A. Pincosy, and R. V. Pyle "Development of R.F. Plasma Generators for Neutral Beams", J. Vac. Sci. Technol. A 3(3), (May/Jun 1985) pp 1218-1221 describes an R.F. plasma source used to generate a high power neutral beam to heat a large fusion plasma to reaction temperatures. The pressure is from 1 to 20 mTorr. A plurality of magnetic dipoles surround the rf bucket chamber to create a magnetic shield for the plasma There is no reference to additive or subtractive treatment of a substrate. Vella et al teaches the use of R.F. induction for plasma production, but it does not teach the use of R.F. induction for etching or deposition where the plasma will etch the coating on an R F. coil and coat the insulators.
R. Limpaecher and K. R. MacKenzie, "Magnetic Multipole Containment of Large Uniform Collisionless Quiescent Plasmas", Rev. Sci. Instrum., Vol 44, No. 6, (June 1973) 726-731 discusses the use of magnetic multipoles for confinement of a plasma of argon at a pressure of 0.002 Torr. This reference is one of the original papers on multipole confinement of the primary electrons in plasma production from electron emission from a hot filament.
U.S. Pat. No. No 4,483,737 of Mantei, "Method and Apparatus for Plasma Etching a Substrate" uses an electrically heated filament to emit electrons, but states at Col. 5, lines 53.65, that a hollow cathode or ion cyclotron resonance can be used to generate electrons. Later, it states that R.F. power sources are not used for the discharge current or for generation of the surface magnetic field confining the plasma. At Col. 6, lines 52-58, it states "The plasma is produced by impact from fast ionizing electrons drawn from a set of heated tungsten filaments, rather than by an applied rf voltage".
See also T. D. Mantei and T. Wicker, "Plasma Etching with Surface Magnetic Field Confinement" Appl. Phys. Lett. 43(1), (1 Jul. 1983) pp. 84-86, and T. D. Mantei and T. Wicker, "Low Pressure Plasma Etching with Magnetic Confinement", Solid State Technology (April 1985) pp. 263-265.
I. Lin, D. C. Hinson, W. H. Class, R. L. Sandstrom, "Low-Energy High Flux Reactive Ion Etching by R.F. Magnetron Plasma", Appl Phys. Lett. Vol. 44 (Jan. 15 1984) pp. 185-187 describes magnetic confinement of a plasma and R.F. power being used for plasma production. The R.F. power is capacitively coupled to a copper prism used as an electrode. This is one of many magnetron reactive ion etching systems. Most of them involve an effort to achieve uniformity from a system in which the electron density increases in the direction of E.times.B drift of secondary electrons from the cathode. These systems also provide limited adjustability of the energy of the ions striking a wafer.
U.S. Pat. No. No. 4,632,719 of Chow et al for "Semiconductor Etching Apparatus with Magnetic Array and Vertical Shield" describes etching a semiconductor wafer in an R.F. field in argon gas A pair of rings of concentric dipoles above the wafer create a pair of rings in the plasma above the wafer. This leads to the kind of lack of uniformity of the plasma which would be avoided in systems required to provide uniform etching or deposition. Thus the Chow et al patent would lead one in the opposite direction from the purpose to which this invention is directed.
U.S. Pat. No. 4,384,938 "Reactive Ion Etching Chamber" of B. Desilets et al describes a reactive ion etching tool having a cylindrical reactive ion etching chamber acting as an anode and a plate arrangement acting as a cathode and wherein an R.F. signal applied between cathode and anode acts to produce an active glow region within the chamber with a dark space existing over the internal surfaces thereof. A reactive ion etching chamber structure has an internal top surface and sidewall surfaces forming a physically symmetrical arrangement with respect to the cathode plate positioned between the sidewall surfaces below the top surface, the top surface and surfaces being uniform except for gas input and exhaust ports with the gas exhaust ports having an opening dimension less than the thickness of the dark space existing over the internal surface.
See also Keller et al U.S. Pat. No. 4,383,177 for "Multipole Implantation Isotope Separation Ion Beam Source".
Objects of this invention are:
a) a uniform plasma, PA1 b) plasma density which does not saturate with power, PA1 c) control of ion energy, PA1 d) high etch rates for a given power level, and PA1 e) relative simplicity.
In accordance with this invention, dry processing apparatus for plasma etching or deposition includes a chamber for plasma processing having an external wall for housing a work piece with a surface to be plasma processed in a gas. A source of an induction field is located outside the chamber on its opposite side from the work piece. A radio frequency (R.F.) induction field applied to the chamber generates a plasma in the gas. The plasma is confined within the external wall in the chamber by magnetic dipoles providing a surface magnetic field for confining the plasma. The surface magnetic field is confined to the space adjacent to the external wall. An R.F generator provides an R.F. generated bias to the work piece. The chamber is lined with a material inert to a plasma or noncontaminating to the work piece, and the induction source in the form of a planar involute or spiral induction coil is located on the exterior of the liner material on the opposite side of the chamber from the work piece. Delivery of and distribution of the gas to the chamber is uniform about the periphery of the top cover because a manifold is located about the periphery of the chamber. An orifice for controlling the gas pressure of the gas being admitted to the chamber is formed by the surface of the chamber and the manifold admits gas from the manifold into the chamber at a uniform pressure about the periphery of the cover of the chamber Preferably a surface magnetic field is positioned adjacent to the induction coil to confine the field at the top of the chamber It is further preferred that a capacitive or inductive reactance be connected in series with the induction coil to adjust the R.F. generated bias.